1. Field of the Invention
The present invention relates to a magnetic disk unit which radiates laser beams onto a recording medium and positions a magnetic head, and a method of the manufacture thereof. Specifically, the present invention relates to a magnetic disk unit for improving the quality of optical sensor signals for positioning a magnetic head, thereby improving positioning accuracy, and a method of the manufacture thereof.
2. Description of Related Art
The present mainstream of recording media attachable to and detachable from magnetic disk units is 3.5-inch disks. The track density of such recording media has reached about 2100 to 2500 TPI (tracks per inch) with the recording capacity of 100 to 120 megabytes. In order to enable recording, erasing, or playing back information of high recording densities, accurate positioning of a magnetic head to a recording medium is essential. Therefore, a recording medium is provided with a servo stitch for detecting the position of an optical tracking servo, and on the positioning of a magnetic head to magnetic tracks, closed-loop optical servo control is performed using the servo stitch for position detecting.
FIG. 7 is a diagram illustrating a conventional magnetic disk unit. FIG. 8 is a diagram illustrating the optical system of the magnetic disk unit shown in FIG. 7. In FIGS. 7 and 8, the numeral 20 indicates a disk recording medium; 20a indicates lands of specific lengths formed concentrically on the bottom surface of the recording medium 20; 20b indicates position detecting servo stitches comprising grooves provided intermittently having a reflection factor different from the reflection factor of the lands 20a; 21 indicates a laser source (hereafter referred to as LD); 22 indicates a laser beam emitted from the LD 21; 23 indicates a 3-beam diffraction grating which splits the laser beam 22 into three beams; 24 indicates an aperture; 25 indicates an objective lens for converging the laser beam 22 from a hologram element 29, and guiding reflected light beams from the recording medium 20 to the hologram element 29; 26 indicates a mirror for guiding the laser beam 22 to the recording medium 20, and guiding light beams reflected from the recording medium 20 to the objective lens 25; 27 indicates a beam splitter; 28 indicates a photodiode (hereafter referred to as PD), which has three light receiving parts 28a-28c; 29 indicates a hologram element comprising the aperture 24 and the beam splitter 27; and 30 indicates a laser source-photodiode unit (hereafter referred to as LD-PD unit) comprising the LD 21, the PD 28, and the hologram element 29.
Also, 31a-31c indicate amplifiers; each of RSM, RS1, and RS2 indicates a feedback resistance; 32 indicates an arithmetic circuit; 32a indicates a driving amplifier; and 33 indicates a voice coil motor for moving a carriage 37.
Furthermore, 34 indicates a magnetic head for recording information on 2the recording medium 20 or playing back information recorded on the recording medium 20; 34a indicates a light path for passing the laser beam and reflected light beams; 35 indicates a magnetic gap of the magnetic head 34; 36 indicates a head support plate for supporting the magnetic head 34; and 37 indicates a carriage for fixing the head support plate 36, and movably supporting the structure comprising components 21-30 together with the magnetic head 34.
The operation of this unit will be described below referring to FIGS. 7 and 8. The recording medium 20 is rotated by a medium driving motor (not shown) at a constant speed. The magnetic head 34 is supported by the head support plate 36, and the magnetic gap 35 slides on the bottom surface of the recording medium 20.
FIG. 8 is a conceptual diagram illustrating sensing the tracking information of the magnetic disk unit and illustrating a closed-loop optical servo control. The laser beam 22 emitted from the LD 21 passes through the 3-beam diffraction grating 23, and split into three laser beams 22a, 22b, and 22c, which pass through the aperture 24 and enter in the objective lens 25. Laser beams 22a, 22b, and 22c, which have passed through the objective lens 25, are reflected from mirror 26 and radiated onto the bottom surface of the recording medium 20 perpendicularly, and form three corresponding beam spots M, S1, and S2 on the surface of the recording medium 20. At this time, the optic axis of the laser beam 22 emitted from the LD 21 is in parallel to the recording medium 20.
Here, if the laser beam 22 parallel to the recording medium 20 emitted from the LD 21 is radiated onto the recording medium 20 perpendicularly using a mirror 26, the adjustment of the optic axis is difficult because this principle acts with light beams reflected by the mirror 26. Therefore, corresponding the fluctuation of the angle of the mirror 26 and the angle of the laser beam 22 radiated onto the bottom surface of the recording medium 20, the adjustment for optimizing the quantity of laser beams that return to light receivers 28a to 28c by aligning the LD-PD unit 30 shown in FIG. 7 in X and Y directions.
Since the light path of the laser beam 22 can be set long by using mirror 26 regardless of the limitation of the thickness of the magnetic disk unit, the effective diameter of the objective lens 25 for achieving the beam-spot diameters xcfx86M, xcfx86S1, and xcfx86S2 can be expanded, and the quantity of light into light receivers 28a to 28c can be increased.
As FIG. 8 shows, servo stitches 20b, which represent information, are formed on the bottom surface of the recording medium 20. The magnetic disk unit senses the location from difference in the quantity of reflected light from beam spots M, S1, and S2 in terms of reflection factors between the land 20a on the bottom surface of the recording medium 20 where no locating servo stitches 20b are present and the locating servo stitches 20b. Three reflected light beams from the recording medium 20 (shown by dotted line in FIG. 8) enter into the objective lens 25. Since the optical system is a non-telecentric system, the three reflected light beams after passing through the objective lens 25 do not necessarily pass through the center of the aperture 24, and are guided by the beam splitter 28 to light receivers 28a to 28c. 
Although the three reflected light beams are received by the light receivers 28a to 28c respectively, they not always pass through the center of the aperture 24. Therefore, the light-beam receiving ratios of the light receivers 28a to 28c vary according to the angle of the mirror 26 and the angle of the laser beam 22 radiated to the bottom surface of the recording medium 20. In order to optimize the quantity of the laser beams returning to the light receivers 28a to 28c, the quantity of light beams are adjusted so that the quantity of light beams received by the light receiver 28a corresponding to the beam spot M is maximized, and the quantities of light beams received by the light receivers 28b and 28c corresponding to the beam spots S1 and S2 are equalized.
FIG. 9 shows the relationship between the servo stitch 20b, Beam spots M, S1, and S2, and the output of PD 28. As FIG. 9 shows, tracking information, that is the position data in the radial direction of the recording medium 20, is determined by the quadrature phase method through the use of the output values of the light receivers 28a to 28c when the beam spots M, S1, and S2 traverse the servo stitch 20b in the radial direction. The output waveform of PD 28 at this time must be sinusoidal waves, and for this reason each of the beam spot diameters xcfx86M, xcfx86S1, and xcfx86S2 is optimized according to the pitch P of the servo stitch 20b. This optimization depends on the diameter of the circular aperture 24.
Furthermore, in order to determine the tracking position information, outputs EM, ES1, and ES2 after amplification by amplifiers 31a to 31c corresponding to the outputs of the receivers 28a to 28c must be equalized. The outputs EM, ES1, and ES2 can be equalized by making the ratio of resistances RM, RS1, and RS2 the reciprocal ratio of the beam splitting ratio of the laser beam 22 by the 3-beam diffraction grating 23. In non-telecentric system, on the other hand, the adjustment of optical axes and the adjustment of positions of the receivers 28a to 28c are required for passing through the aperture 24 the reflected light beams of beam spots S1 and S2 from the recording medium 20 evenly as described above.
Tracking information can be obtained from the results of outputs EM, ES1, and E2 computed based on the principle of the quadrature phase method, and transmitted to a driving amplifier 32a for driving the voice coil 33. A current corresponding to the error of the tracking position drives the voice-coil motor 33, and a magnetic gap 35 maintaining a certain distance to beam spots M, S1, and S2 is positioned on a specific track. By this, closed loop optical servo control is carried out.
The magnetic disk unit is adjusted when manufactured so that the direction of three laser beams 22a to 22c form predetermined angles against the position sensing servo switch 20b when the three laser beams 22a to 22c are radiated to the position sensing servo switch 20b. 
Since conventional magnetic disk units are constituted as described above, the beam spot diameters xcfx86M, xcfx86S1, and xcfx86S2 of three laser beams must be adjusted to form images accurately on the bottom surface of the recording medium 20. The accuracy of the laser beams in the optical axis direction, or focussing accuracy, must be xc2x150 xcexcm, and the adjustment of focussing requiring the accuracy of xc2x150 xcexcm is normally carried out by adjusting the position of the objective lens 25. However, since the hologram element 29, the objective lens 25, the mirror 26, and other elements are independent parts, the relative position of each part must be delicately adjusted. Furthermore, the tilt of the objective lens 25 must be taken into consideration, making focus adjustment difficult.
In order that the light receivers 28a to 28c sense reflected light beams accurately, the alignment of the optical axis of the laser beam 22 to the position sensing servo switch 20a is required. However, this operation is also difficult because the LD-PD unit 30, the objective lens 25, and the mirror 26 are independent parts. Also, since the objective lens 25 is non-telecentric, the reflected light beams from the recording medium 20 do not always pass through the center of the aperture 24, resulting in the imbalance of outputs from the light receivers 28a to 28c. Therefore, in order to make EM maximum and to equalize ES1 and ES2 finally, the position adjustment of each part, in particular of the LD-PD unit 30 is essential.
In the structure to make laser beams 22 parallel to the surface of the recording medium 20 traverse perpendicularly to the surface of the recording medium 20, the accuracy of installation angle of the mirror 26 becomes strict due to the principle of optical lever, which, together with the effect of the non-telecentric optical system, makes difficult the adjustment of sensing accuracy by the light receivers 28a to 28c. 
Since the LD-PD unit 30, the objective lens 25, the mirror 26, and the like elements are independent parts, the adjustment of positions and the reduction of size and weight of the optical system are difficult, and the servo properties of magnetic disk units cannot be improved.
The present invention solves the problems described above. It is an object of the present invention to provide a magnetic disk unit by which a sufficient output of laser beams for optical tracking. It is another object of the present invention to make laser beams traverse perpendicularly to a recording medium without using mirrors for simplifying tilt adjustment. It is another object to provide a magnetic disk unit which can simplify or eliminate focussing adjustment and the like. It is a further object of the present invention to improve the servo properties of magnetic disk units through the weight reduction of the optical system.
According to a first aspect of the present invention, there is provided a magnetic disk unit for optically sensing tracking information recorded on a disk recording medium, thereby positioning a magnetic head to a predetermined recording track on the recording medium, and magnetically recording information on the recording medium, or playing back recorded information, the magnetic disk unit comprising: an optical means installed underneath the magnetic head for radiating a laser beam onto the recording medium through a light path provided in the magnetic head and receiving the reflected beam, the optical means having: a laser source, a diffraction grating for splitting the laser beam from the laser source into a plurality of beams, an aperture for controlling the cross-sections of a plurality of laser beams from the diffraction grating and reflected light beams from the recording medium, an objective lens for converging a plurality of laser beams from the aperture and irradiating the recording medium as well as guiding the reflected light beams from the recording medium to the aperture, a beam splitter for splitting the reflected light beams transmitted through the aperture, and a light receiving member for receiving light beams split by the beam splitter, wherein the object lens being an objective lens telecentric toward the image surface.
According to a second aspect of the present invention, there is provided a method of manufacturing a magnetic disk unit for optically sensing tracking information recorded on a disk recording medium, thereby positioning a magnetic head to a predetermined recording track on the recording medium, and magnetically recording information on the recording medium, or playing back recorded information, the magnetic disk unit comprising: an optical means installed underneath the magnetic head for radiating a laser beam onto the recording medium through a light path provided in the magnetic head and receiving the reflected beam, the optical means having: a laser source, a diffraction grating for splitting the laser beam from the laser source into a plurality of beams, an aperture for controlling the cross-sections of a plurality of laser beams from the diffraction grating and reflected light beams from the recording medium, an objective lens for converging a plurality of laser beams from the aperture and irradiating the recording medium as well as guiding the reflected light beams from the recording medium to the aperture, a beam splitter for splitting the reflected light beams transmitted through the aperture, and a light receiving member for receiving light beams split by the beam splitter, wherein the object lens being an objective lens telecentric toward the image surface, wherein the laser source, the diffraction grating, the aperture, the objective lens and the recording medium are arranged in a layered structure in the height direction, and the distance between the upper surface of the objective lens and the recording medium is a predetermined length, the method comprising the steps of: radiating referent parallel light beams passing through the aperture from the side of the laser source for allowing the center of the aperture to coincide with the center of the objective lens; and assembling the aperture and the objective lens so that the center of the aperture coincides with the image-forming light from the objective lens.
According to a third aspect of the present invention, there is provided a method of manufacturing a magnetic disk unit for optically sensing tracking information recorded on a disk recording medium, thereby positioning a magnetic head to a predetermined recording track on the recording medium, and magnetically recording information on the recording medium, or playing back recorded information, the magnetic disk unit comprising: an optical means installed underneath the magnetic head for radiating a laser beam onto the recording medium through a light path provided in the magnetic head and receiving the reflected beam, the optical means having: a laser source, a diffraction grating for splitting the laser beam from the laser source into a plurality of beams, an aperture for controlling the cross-sections of a plurality of laser beams from the diffraction grating and reflected light beams from the recording medium, an objective lens for converging a plurality of laser beams from the aperture and irradiating the recording medium as well as guiding the reflected light beams from the recording medium to the aperture, a beam splitter for splitting the reflected light beams transmitted through the aperture, and a light receiving member for receiving light beams split by the beam splitter, wherein the object lens being an objective lens telecentric toward the image surface, wherein the laser source, the diffraction grating, the aperture, the objective lens and the recording medium are arranged in a layered structure in the height direction, and the distance between the upper surface of the objective lens and the recording medium is a predetermined length, the method comprising the step of: assembling the aperture and the objective lens so that the center of the aperture offsets from the center of the objective lens depending upon the angle between the bottom surface of the magnetic head and the recording media sliding surface.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of the embodiments thereof taken in conjunction with the accompanying drawings.